1. Field of the Invention
The present invention relates to a magnetic recording medium, particularly to a floppy disk having a high content of magnetic powder and a low surface electrical resistance of a magnetic layer.
2. Description of Related Art
Recently, the development of a floppy disk (referred to as "FD" hereinafter) is remarkably proceeded and it has been is required that the floppy disk have a higher recording density and a larger capacity. With an increase of capacity, high reliability of recording/reproduction is strongly required. One of factors deteriorating the reliability of the FD is adhesion of dust to the magnetic layer surface. The degree of adhesion of dust depends on the surface electrical resistance of the magnetic layer. When the magnetic layer has a high surface electrical resistance, the magnetic layer surface is easily charged so that the dust in the air easily adheres to the surface. The adhered dust is a main reason for the missing pulse (MP). In order to increase the reliability of the FD, the magnetic layer should have a low surface electrical resistance (preferably smaller than 5.times.10.sup.7 .OMEGA./.quadrature.).
In the prior art, the magnetic layer frequently contains carbon black so as to decrease the surface electrical resistance of the magnetic layer. The volume fraction of carbon black should be large enough to provide a sufficiently low surface electrical resistance. However, the increase of the volume fraction of carbon black gives an unsuitable decrease in the volume fraction of the magnetic powder in view of the electromagnetic conversion property. Thus, carbon black is excluded from the magnetic layer, and an electrically conductive anchor layer (electrically conductive resin layer) containing carbon black is formed under the magnetic layer so that the surface electrical resistance of the magnetic layer is decreased. In addition, a coated electroconductive polymer (for example, polyaniline) layer is substituted for the electrically conductive anchor layer. However, the electroconductive polymer layer has poor adhesion to a polyethylene terephthalate (PET) base film, and it is necessary to form a firm grip layer between the electroconductive polymer layer and the base film, which firm grip layer facilitates a firm connection of the electroconductive polymer layer with the base film. The multiple layer-coating of the firm grip layer, the electroconductive polymer layer and the magnetic layer decreases the productivity, gives a low yield and increases the cost of the recording medium. Therefore, in order to decrease the surface electrical resistance of the magnetic recording medium, a new technology was sought which is easy, has high productivity and decreases the production cost. On the other hand, it is not necessary to make an effort so as to decrease the surface electrical resistance of the magnetic layer of a video tape having a back coating layer. The reason is that the back coating layer containing carbon black has a sufficiently low electrical resistance and consequently the magnetic layer on the opposite side has a low surface electrical resistance when the base film is thin. However, for the magnetic recording medium having two magnetic layers between which the base film is present, such as the FD, it is necessary to decrease the electrical resistance of the magnetic layer itself. The inherent electrical resistance of Ba ferrite magnetic powder itself used for the FD having high recording density and large capacity is remarkably higher than that of magnetic metal powder. Therefore, the technology decreasing the surface electrical resistance of the magnetic layer is particularly required in the development of the large capacity FD.
In the prior art, a means for remarkably and drastically decreasing the surface electrical resistance of the magnetic layer has not been developed, and there is no literature or patent specification disclosing such means.
In the prior art, the problems of low productivity and high cost cannot be solved. In order to obtain the FD having the high recording density and the large capacity, it is necessary to use a high content of Ba ferrite magnetic powder in the magnetic layer. Thus, the magnetic layer disadvantageously has too high a surface electrical resistance of the magnetic layer.